For the past decade, the Environmental Protection Agency (EPA) and the waste management industry have been promoting the use of processed sewage sludge for farm and garden. Likewise, in communities across the country, waste treatment plants are composting or otherwise treating their sewage sludge and offering it as a soil amendment.
The idea of returning the nutrients in human waste to the soil is not new. For thousands of years, Chinese farmers collected “night soil” to fertilize their fields. Unfortunately, these days human isn’t the only thing that gets flushed down the drain. Schools, hospitals, dry cleaners, and industries of all sorts send their wastewater to the same sewage treatment plant. Pesticides, PCBs, dioxins, radioactive wastes, asbestos, heavy metals, and petroleum compounds are among the pollutants found in sludge once the municipal waste has been treated. Of these pollutants, the EPA has set standards regulating only a handful of contaminants: the heavy metals arsenic, cadmium, chromium, copper, lead, mercury, zinc, nickel, molybdenum, and selenium.
These standards, however, do not appear adequate to protect human health or the health of agricultural lands, according to Cornell’s Waste Management Institute. Some of these heavy metals are toxic to plants at high levels, and certain metals—notably lead, cadmium and mercury—are toxic to animals. Heavy metals can kill soil organisms vital to the decomposition of leaf litter and can harm beneficial nitrogen-fixing bacteria. Unlike the organic components of sludge, heavy metals do not decompose. They remain in the soil, accumulating to levels that eventually make the soil unfit for farming or gardening.